Recently, as portable electronic devices have widely spread, in accordance with miniaturization, thinness, and lightness of the portable electronic devices, research into a secondary battery used as a power source for these portable electronic devices, which may have a small size and a light weight and be charged and discharged for a long period of time, has been actively conducted.
The lithium secondary battery, which generates electrical energy by oxidation-reduction reactions when lithium ions are intercalated into and deintercalated from a cathode and an anode, is manufactured by using a material capable of intercalating and deintercalating lithium ions as the anode and the cathode, and filling an organic electrolyte or polymer electrolyte between the cathode and the anode.
An example of an organic electrolyte widely used at the present time includes ethylene carbonate, propylene carbonate, dimethoxyethane, gamma butyrolactone, N,N-dimethylformamide, tetrahydrofuran, acetonitrile, or the like. However, generally, since the organic electrolyte as described above may be easily volatilized and have high flammability, at the time of applying the organic electrolyte to a lithium ion secondary battery, a safety problem, for example, ignition due to an internal short-circuit when heat is generated in the battery by over-charge or over-discharge, or the like, may occur at a high temperature.
Further, at the time of initial charge of the lithium secondary battery, lithium ions released from a lithium metal oxide, which is a cathode, move to a carbon electrode, which is an anode, to thereby be intercalated into carbon. In this case, since lithium has high reactivity, while a surface of carbon particles, which is an anode active material, and an electrolyte react with each other, a coating film referred to as a solid electrolyte interface (SEI) film is formed on a surface of the anode.
Performance of the lithium secondary battery significantly depends on a configuration of the organic electrolyte and the SEI film formed by a reaction of the organic electrolyte and the electrode.
That is, the formed SEI film may suppress side reactions of a carbon material and an electrolyte solvent, for example, decomposition of the electrolyte on the surface of the carbon particle, which is the anode, prevent disintegration of an anode material caused by co-intercalation of the electrolyte solvent into the anode material, and serve as a lithium ion tunnel according to the related art, thereby minimizing deterioration in performance of the battery.
Therefore, various researches for developing a novel organic electrolyte containing an additive in order to solve the above-mentioned problem have been conducted.
For example, a non-aqueous lithium ion battery capable of preventing over-charge current and a thermal runaway phenomenon caused by the over-charge current by using an aromatic compound such as biphenyl has been disclosed in Japanese Patent No. 2002-260725. In addition, a method of improving safety of a battery by adding a small amount of an aromatic compound such as biphenyl, 3-chlorothiophene, or the like, to increase an internal resistance by electrochemical polymerization in an abnormal over-voltage state has been disclosed in U.S. Pat. No. 5,879,834. However, in the case of using the additive such as biphenyl, or the like, there are problems in that when a relatively high voltage is locally generated in a general operation voltage, the additive is gradually decomposed during a charge and discharge process, or when the battery is discharged at a high temperature for a long period of time, an amount of biphenyl, or the like, may be gradually decreased, such that safety may not be secured after 300 charge and discharge cycles. In addition, there is a problem in storage characteristics, or the like.
Therefore, research for improving safety at high and low temperatures while still having a high capacity retention rate has been continuously demanded.